The present invention relates to making axisymmetric parts of varying diameter out of composite material, and more particularly relates to fiber structures for such parts.
An example application of the invention lies in making aircraft wheels (or rims). Another example application lies in making compressor casings for helicopter gas turbine engines.
In known manner, an aircraft wheel is made up of two portions that are fastened together so as to enable the tire to be put on the wheel and removed therefrom. Each of the two wheel-halves is in the form of an open ring of C-shaped section. The wheel also receives a brake system made up in particular of a brake disk made of composite material and housed in one of the wheel-halves.
Such aircraft wheels are typically made of aluminum, each wheel-half being obtained by forging, for example. Nevertheless, making wheels out of metal increases their weight and requires a large amount of maintenance, largely because of galvanic corrosion between the metal wheels and the brake disks made of composite material.
In order to mitigate such a drawback, proposals have been made to make the wheel-halves making up an aircraft wheel out of composite material comprising fiber reinforcement densified by a matrix. More precisely, the fiber reinforcement of a wheel-half is formed by a fiber structure that can be woven or braided as a single piece by so-called “contour weaving” on a forming mandrel having a portion of frustoconical shape. Various geometrical transformations are then applied to the fiber structure in order to bring it into the shape of an open ring of C-shaped section, corresponding to a fiber preform for an aircraft wheel-half. The fiber preform as obtained in this way is then densified by a matrix in order to form an aircraft wheel-half, e.g. by using resin transfer molding (RTM). Reference may thus be made to the French application No. 12/58720 filed on Sep. 18, 2012, which describes implementations of such a fabrication method.
Making a fiber preform for an aircraft wheel-half by means of such a method is perfectly possible when the ratio between the large diameter and the small diameter of the frustoconical portion of the fiber structure woven or braided on the forming mandrel is close to 1. With such a ratio, it is possible to obtain a fiber content of the order of 55% to 60% that is uniform throughout the preform, thereby imparting thereto structural strength that is satisfactory for the intended application.
In contrast, when the ratio between the large diameter and the small diameter of the frustoconical portion of the fiber structure is not less than 2, it becomes difficult using the method of French patent application No. 12/58720 to obtain a sufficiently high fiber content that is uniform throughout the preform. The increase in the ratio between the diameters of the fiber structure has the consequence of increasing the spacing between adjacent weft yarns in the large diameter thereof. This results in a fiber content that is not uniform throughout the preform.
In order to remedy that problem, it is possible to provide additional weft yarns in the large diameter zone and to interrupt them going towards the small diameter zone. Nevertheless, that solution is not satisfactory for the strength of the part that is made.